For example, two seawater desalination plants coupled with two power plants are recently installed in the Arabian Gulf area. The desalination plants are nearly identical and based on the Recycle-Brine Multi-Stage Flash (RB-MSF) concept. Each plant consists of eight identical trains, and each train produces 15 million U.S. gallons per day (MGD) of distillate at 100% load. The distillate production capacity of each plant is thus 120 MGD. FIG. 1 shows a simplified flow diagram for each BR-MSF train. The train mainly consists of three major sections: (1) heat rejection section; (2) heat recovery section; and (3) brine heater. The number of stages in the heat rejection section is 3 while the number of stages in the heat recovery section is 20.
Seawater is pretreated by mainly screening and chlorination before entering the last stage of the heat rejection section through heat transfer tubing (condensers/pre-heaters). A significant volume of pretreated seawater is required for cooling in the heat rejection section. About 70% of the preheated seawater exiting the heat rejection section is rejected as “returned cooling seawater”. About 20% of such returned cooling seawater is recycled for mixing with seawater feed stream to maintain a constant temperature at the entrance of the heat rejection section whereas the remaining returned cooling seawater is mixed with reject brine from the last stage of the heat rejection section before blowing it back to the sea.
The remaining portion of the preheated seawater existing the heat rejection section (about 30% of the total required seawater) is mixed with additives (e.g., anti-scale and anti-foam) and fed to a vacuum de-aerator. The preheated and de-aerated seawater is then mixed with a portion of the flashed off brine from the last stage of the heat rejection section to form recycle brine. An oxygen scavenger is added to the recycle brine before passing it to the last stage of the heat recovery section.
In the heat recovery section, recycle brine passes through heat transfer tubing (condensers/pre-heaters) before entering the brine heater. The brine heater is externally driven by low-pressure (LP) and intermediate-pressure (IP) steam to heat recycle brine to the desired or designed top brine temperature of the train.
The heated recycle brine flows from the brine heater to the first flashing stage where the pressure is lowered so that it is just below the vapor saturation pressure of water. This sudden introduction of heated recycle brine into a lower pressure stage causes it to boil so rapidly as to flash into vapor. However, a small fraction of the recycle brine is converted into vapor. The remaining recycle brine thus passes through a series of flashing stages, each possessing a lower pressure to lower the boiling point of recycle brine than the previous stage. This allows consecutive reduction of the boiling point of recycle brine as it gets more concentrated in going down the flashing stages. MSF is thus a multiple boiling concept without supplying additional heat after the brine heater.
The flashed off vapor condenses on the tubes side of the condensers/pre-heaters and transports across the heat recovery section as distillate. The released latent heat of the condensed vapor is used to preheat the incoming recycle brine in the heat recovery section. Since the colder recycle brine entering the heat recovery section counter flows with the flashed off brine/distillate, relatively little heat energy leaves in the flashed off brine from the last stage of the heat recovery section. The unflashed portion of recycle brine then passes through additional flashing stages in the heat rejection section to recover more distillate. As such, most of the heat energy of the flashing process is exchanged with the recycle brine (heat recovery section) and seawater (heat rejection section) flowing into the train.
However, thermally unstable ions (bicarbonate, magnesium and calcium) in seawater present engineering difficulties that severely limit the potential capacity of any desalting method. In the RB-MSF train, bicarbonate in seawater is thermally but partially broken down in pre-heaters (heat rejection section) into carbon dioxide and hydroxides. Carbon dioxide is removed from seawater by the vacuum de-aerator whereas the released hydroxides along with the gradual temperature rise of seawater (heat rejection section) and recycle brine (heat recovery section) promote magnesium hydroxide (brucite) scaling. The initiation of brucite scale is critical since it acts as nucleation sites for calcium-containing scale species (carbonate in the form of aragonite and sulfates).
Calcium has three forms of sulfate. Calcium sulfate anhydrous and/or hemihydrate scaling takes place at higher temperatures (near the top brine temperature) whereas calcium sulfate dihydrate (gypsum) scaling takes place at lower temperatures. The solubility limits of calcium sulfate anhydrous or hemihydrate are inversely and steeply proportional with temperatures. Scale inhibitors are added mainly to delay the precipitation of sulfate scales even though their effect is very limited in solving such scales especially within the front end flashing stages in the heat recovery section. The temperature tolerance limit of scale inhibitors also dictates the top brine temperature (e.g., 90° C. for polyphosphates and 110° C. for polycarboxylates or polymeric), which is an undesirable restriction.
The desalination plants (I and II) are unusually located near a marshy shallow seawater area. Seawater can not be drawn from a deep water column to take advantage of reduced oxygen, suspended solids and microbial activity at depth. Seawater in that area is also known of having high silt index and low natural current. As shown in Table 1, the consequences of the plants' location along with discharging back to the sea a copious volume of reject brine (905.4 MGD) from both plants are clearly pronounced in the differences of the TDS and sulfate levels before and after operating the plants. Sulfate level, for example, has surged from 2,700 mg/L in 2006 to 4,100 mg/L in 2013 (increased by 52%).
Table 2 shows different operating conditions for a given RB-MSF train to cope with the consequences of improper plants' location. FIG. 2 reveals that the concentration of calcium sulfate anhydrous in the front-end flashing stages close to the brine heater (stage 1 to stage 4) when seawater has a normal sulfate level (2,700 mg/L) in 2006 is slightly below the saturation line. If the same operating conditions of the RB-MSF train as built including the same concentration factors (recycle brine/seawater and reject brine/seawater) are held regardless of the changes in seawater composition (4,100 mg/L of sulfate level in 2013), the concentration of calcium sulfate anhydrous in the flashing stages close to the brine heater is over saturated (FIG. 2). As presented in Table 2, the TDS and sulfate levels are appreciably increased in recycle brine and reject brine (e.g., the TDS and sulfate surges in recycle brine are, respectively, from 56,101 to 70,943 mg/L and from 3,835 to 5,823 mg/L).
In order to operate the RB-MSF plants properly and manage the detrimental changes in seawater composition (Table 1) in the form of sulfate scales, the reject brine concentration (CB) must be fixed at 63,200 mg/L of TDS as the plants originally designed. The concentration factor of recycle brine/seawater (Cr/CF) would be reduced to 1.13 from 1.42 whereas the concentration factor of reject brine/seawater (CB/CF) would be reduced to 1.27 from 1.6 (Table 2). The concentration of calcium sulfate anhydrous in the flashing stages close to the brine heater would then be kept at the saturation border line as shown in FIG. 2. However, the required seawater for blending with flashed off brine to form recycle brine would increase by 75% (Table 2). This, in turn, would: (1) elevate the required amounts of additives (anti-foam, anti-scale, and oxygen scavenger); (2) require larger vacuum de-aerators; (3) substantially increase the volume of reject brine (by 121%); and (4) require higher pumping power. It should be pointed out that the total required seawater (returned cooling seawater and preheated seawater to be blend with flashed off brine from the heat rejection) remains the same as in the original RB-MSF design (Table 2); returned cooling seawater is just proportionally decreased with the increase of preheated seawater to feed the heat recovery section.
Recycle Brine (RB) became the conventional approach compared to the Once-Through (OT) approach in designing MSF plants in the past 20 years for several presumed reasons. First, recycle brine reduces the actual volume of seawater to feed the trains for distillate production, which would reduce the amounts of additives, size of vacuum de-aerators and volume of reject brine. Second, RB-MSF adds a heat rejection section in each train to: (1) recover more distillate at the low flashing range (40-33° C.); (2) maintain a constant temperature at the entrance of the heat rejection section by blending a portion of returned cooling seawater with seawater feed stream, (3) dilute reject brine with the remaining portion of the returned cooling seawater before blowing it down to the sea; and (4) eliminate seawater de-alkalization. Seawater pretreatment in desalination plants conventionally includes a de-alkalization step by dosing an acid (e.g., sulfuric acid) to convert bicarbonate to carbon dioxide, removing carbon dioxide by the de-aerator, and then neutralizing the pretreated seawater or distillate (e.g., with caustic soda) to re-adjust the pH.
On the other hand, the OT-MSF is a simpler approach than the RB-MSF approach since it consists of a number of stages and a brine heater as shown in FIG. 3. The performance ratio (PR) of OT-MSF train is comparable to RB-MSF train for the same number of stages and top brine temperature. Reject brine from OT-MSF train has low TDS gains compared to seawater and is directly discharged to the sea. As such, OT-MSF train does not require a substantial volume of cooling seawater since it has no heat rejection section nor a substantial pumping power to circulate a tremendous volume of recycle brine as is the case with RB-MSF plant. However, OT-MSF plant has a lower ratio of distillate/seawater than RB-MSF plant (no recycle brine), and it is brine from the last stage is typically rejected at 40° C. rather than 33° C. (a further but slight loss of distillate recovery), which are considered the drawbacks of the OT-MSF approach.
When an OT-MSF train, however, is applied to the changed composition of seawater (Table 1) by fixing CB at 63,200 mg/L of TDS as the RB-MSF train originally designed, it provided better performance than RB-MSF train with distinct advantages (Table 2). Such advantages can be seen in: (1) reducing the steam load (from 62.9 to 33.0 kg/s) and therefore substantially increasing the PR (from 9.4 to 13.6); (2) entirely eliminating the substantial volume of cooling seawater; and (3) keeping the concentration of calcium sulfate anhydrous in the flashing stages close to the brine heater slightly below the saturation border line (FIG. 2).
Regardless of the type of MSF plants, whether they are based on RB or OT approach, in order to produce 240 MGD of distillate and operate the plants properly, the saturation envelop of calcium sulfate anhydrous must be controlled by fixing CB at 63,200 mg/L of TDS. As consequences, the volumes of the total required seawater (2,247 MGD for RB-MSF and 1,146 MGD for OT-MSF) and reject brine (905.4 MGD for both RB-MSF and OT-MSF) are enormous. Thermal energy to brine heaters can be reduced from one-half to two thirds by pairing with waste heat from the turbines of the coupled power plants and at the same time providing cooling for the power plants, but pumping energy to circulate enormous volumes of seawater, recycle brine and reject brine along with pre-treating a large volume of seawater have become the largest operating costs of MSF plants. The discharge of a copious volume of reject brine to the sea increases TDS around seawater intake lines since it is dispersion not fast enough (shallow seawater, low natural current, and insufficient or absent mechanical dispersion devices), which deteriorates the natural composition of seawater and imposes different sets of plants' operating conditions. Disposal of reject brine has also a significant environmental impact on marine habitat including depleted oxygen, residue of the deoxygenating agent, concentrated toxic species (e.g., derivatives of boron and chlorine), and induction of gypsum precipitation. Such enormous engineering, economic and environmental difficulties are solely caused by calcium sulfate scale; a serious obstacle to the development of low cost distillate production from seawater desalination plants.
Effective de-scaling is thus essential in seawater desalination since it would: (1) allow better performance ratio (ratio of distillate to heating steam) and thus more efficient and cost effective plants; (2) reduce the volumes of seawater (substantial savings on pumping power and pre-treatment) and reject brine; (3) eliminate the addition of scale inhibitors; (4) allow desalination plants such as MSF to reach a higher range of top brine temperatures (120-150° C.) and thus increase their performance ratio by either decreasing steam load or increasing distillate production; (5) prevent maintenance shut-downs for chemical and mechanical scale removal; and (6) slow the decrease of heat transfer coefficients.
Thus, the first objective of this invention is to effectively eliminate scale issues in any desalination plants to achieve the above mentioned benefits. As a result of effective de-scaling, the second objective of this invention is to develop a new effective design for MSF plants, which is based on what I coined Brine-Forward (BF), to entirely eliminate brine recycling and returned cooling seawater.